Yii 1.1 supports default php session handler (CHttpSession) and SQL database session storage (CDbHttpSession). So, this extension contains and interface that you can use to implement any type of session storage without rewriting of HttpSession component methods. Also you session storage will contain optional extra parameter of session - "userId", so you will be able to distinguish guests' sessions and authenticated users' sessions. Another problems which solves this extension described here (in can be very useful):
http://blog.jviba.com/en/web/asynchronous-change-of-session-in-php